Implement gitlab_issue_search MCP Tool and service

Proposal

Add support for new MCP tool: gitlab_issue_search which maps to Search API - Issues

• Add a new API mapping following our framework/annotation approach
• Allow the following arguments from the API documentation
  • scope=issues (required)
  • search (required)
  • confidential (optional)
  • order_by (optional)
  • sort (optional)
  • state (optional)
  • fields (optional)

route_setting :mcp, tool_name: :gitlab_issue_search, params: [:search]